It is becoming increasingly necessary to accurately measure gas compositions and in particular to accurately measure transient engine exhaust emissions. Exhaust gas emissions are generally measured by either a chemiluminescent or a non-dispersive infrared analyzer. The amount of water vapor in an exhaust gas sample can be a significant source of error affecting the measurement of engine exhaust emissions by both types of analyzers. Diesel engine exhaust gas, for example, may contain as much as 15% water vapor by volume. If an atmospheric chamber chemiluminescent analyzer is used to measure nitric oxide (NO) in diesel engine exhaust gas, water vapor will negatively affect the operation of the analyzer and, for a wet exhaust gas sample containing 15% water vapor, the indicated amount of NO may be as much as 30% below the actual amount present in the sample. In contrast, water vapor in the exhaust gas will positively influence the analysis mechanism in an uncompensated non-dispersive infrared analyzer resulting in an indicated NO measurement higher than the actual amount of NO present in the sample.
In response to the above problem, a number of attempts have been made to establish correction factors for specific analyzers and gas samples having a known water vapor content. One approach has been to pass a dry gas of a known composition through a conventional water bubbler. In this method, the bubbler temperature, partial pressure, number of stages, and gas flow rate are varied to produce a desired saturation level in the gas. However, in passing gas compositions containing nitric oxide (NO) through a water bubbler, some of the NO may convert to NO.sub.2 and combine with the water in the bubbler. Further, the amount of loss of NO from the test gas varies with the flow rate and saturation level of the gas. The composition of the resultant gas is thereby undesirably altered, negating the establishment of a predetermined correction factor.
In a second method for controlling the water vapor content of a gas containing NO.sub.2 or other water-reactive components, a non-reactive carrier gas such as nitrogen or argon is saturated by passing through one or more water bubbler stages and then combined with the test gas. This method is described in a report titled Chemiluminescent Measurement of Nitric Oxide in Combustion Products by Blair A. Folsom and Craig W. Courteney, presented March, 1979 at the Third Stationary Source Combustion Symposium sponsored by the EPA Office of Research and Development. This method, while being effective in preventing dilution of NO in the test gas, has the inherent disadvantage of a limited saturation level in that only a portion of the resultant gas passes through the bubbler stages.
A less conventional approach has been to reactively combine hydrogen and oxygen to form water in the test gas. However, this method is not compatible with all gas compositions.
The present invention is directed to overcoming one or more of the problems as set forth above. More particularly, the present invention provides both an apparatus and a method for controllably saturating a gas without undesirably altering the composition of the gas.